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      <h1 id="Java-基础-String-StringBuilder-StringBuffer"><a href="#Java-基础-String-StringBuilder-StringBuffer" class="headerlink" title="Java-基础-String,StringBuilder,StringBuffer"></a>Java-基础-String,StringBuilder,StringBuffer</h1><h2 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h2><ul>
<li>java中<code>String、StringBuffer、StringBuilder</code>是编程中经常使用的字符串类，他们之间的区别也是经常在面试中会问到的问题。现在总结一下，看看他们的不同与相同。</li>
</ul>
<p><img src="http://zhuuu-bucket.oss-cn-beijing.aliyuncs.com/img/20200811-145020237.png" alt="mark"></p>
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<h2 id="1-可变和不可变"><a href="#1-可变和不可变" class="headerlink" title="1. 可变和不可变"></a>1. 可变和不可变</h2><p><strong>1.可变与不可变</strong></p>
<p>　　<code>String</code>类中使用字符数组保存字符串，如下就是，因为有“final”修饰符，所以可以知道string对象是不可变的。</p>
<p>　　　　<strong>private final char value[];</strong></p>
<p>　　<code>StringBuilder与StringBuffer</code>都继承自<code>AbstractStringBuilder</code>类，在<code>AbstractStringBuilder</code>中也是使用字符数组保存字符串，如下就是，可知这两种对象都是可变的。</p>
<p>　　　　<strong>char[] value;</strong></p>
<h3 id="1-1-你见到的字符串被改变的？？？（这是一个假象）"><a href="#1-1-你见到的字符串被改变的？？？（这是一个假象）" class="headerlink" title="1.1 你见到的字符串被改变的？？？（这是一个假象）"></a>1.1 <strong>你见到的字符串被改变的？？？（这是一个假象）</strong></h3><ul>
<li>简要的说， String 类型和 StringBuffer 类型的主要性能区别其实在于 <strong>String 是不可变的对象, 因此在每次对 String 类型进行改变的时候其实都等同于生成了一个新的 String 对象，然后将指针指向新的 String 对象，这样不仅效率低下，而且大量浪费有限的内存空间，所以经常改变内容的字符串最好不要用 String 。</strong>因为每次生成对象都会对系统性能产生影响，特别当内存中无引用对象多了以后， JVM 的 GC 就会开始工作，那速度是一定会相当慢的。</li>
</ul>
<p><img src="http://zhuuu-bucket.oss-cn-beijing.aliyuncs.com/img/20200811-144909950.png" alt="mark"></p>
<ul>
<li>我们可以看到，初始String值为“hello”，然后在这个字符串后面加上新的字符串“world”，这个过程是需要重新在栈堆内存中开辟内存空间的，最终得到了“hello world”字符串也相应的需要开辟内存空间，这样短短的两个字符串，却需要开辟三次内存空间，不得不说这是对内存空间的极大浪费。为了应对经常性的字符串相关的操作，就需要使用Java提供的其他两个操作字符串的类——StringBuffer类和StringBuild类来对此种变化字符串进行处理。</li>
</ul>
<h2 id="2-多线程安全"><a href="#2-多线程安全" class="headerlink" title="2. 多线程安全"></a>2. 多线程安全</h2><ol>
<li><p><code>String</code>中的对象是不可变的，也就可以理解为常量，<strong>显然线程安全</strong>。</p>
</li>
<li><p><code>AbstractStringBuilder</code>是<code>StringBuilder</code>与<code>StringBuffer</code>的公共父类，定义了一些字符串的基本操作，如expandCapacity、append、insert、indexOf等公共方法。</p>
</li>
<li><p><code>StringBuffer</code>对方法加了同步锁或者对调用的方法加了同步锁，所以是<strong>线程安全的</strong>。看如下源码：</p>
</li>
</ol>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">synchronized</span> StringBuffer <span class="title">reverse</span><span class="params">()</span> </span>&#123;</span><br><span class="line">    <span class="keyword">super</span>.reverse();</span><br><span class="line">    <span class="keyword">return</span> <span class="keyword">this</span>;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">indexOf</span><span class="params">(String str)</span> </span>&#123;</span><br><span class="line">    <span class="keyword">return</span> indexOf(str, <span class="number">0</span>);        <span class="comment">//存在 public synchronized int indexOf(String str, int fromIndex) 方法</span></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>



<ol start="4">
<li>StringBuilder并没有对方法进行加同步锁，所以是<strong>非线程安全的</strong>。</li>
</ol>
<h2 id="3-StringBuilder和StringBuffer共同点"><a href="#3-StringBuilder和StringBuffer共同点" class="headerlink" title="3. StringBuilder和StringBuffer共同点"></a>3. StringBuilder和StringBuffer共同点</h2><ul>
<li>StringBuilder与StringBuffer有公共父类AbstractStringBuilder(<strong>抽象类</strong>)。</li>
<li><strong>抽象类与接口的其中一个区别</strong>是：抽象类中可以定义一些子类的公共方法，子类只需要增加新的功能，不需要重复写已经存在的方法；而接口中只是对方法的申明和常量的定义。</li>
<li><code>StringBuilder、StringBuffer</code>的方法都会调用<code>AbstractStringBuilder</code>中的公共方法，如<code>super.append</code>(…)。只是StringBuffer会在方法上加synchronized关键字，进行同步。</li>
</ul>
<h2 id="4-字符串常量池"><a href="#4-字符串常量池" class="headerlink" title="4. 字符串常量池"></a>4. 字符串常量池</h2><ul>
<li>Java中的字符串常量池（<code>String Pool</code>）是存储在Java<strong>堆内存中的字符串池</strong>。我们知道String是java中比较特殊的类，我们可以使用new运算符创建String对象，也可以用双引号（”“）创建字串对象。</li>
</ul>
<p><img src="http://zhuuu-bucket.oss-cn-beijing.aliyuncs.com/img/20200811-151808077.png" alt="mark"></p>
<ul>
<li>之所以有字符串常量池，是因为String在Java中是不可变（immutable）的，它是String interning概念的实现。字符串常量池也是亨元模式（Flyweight）的实例。</li>
<li>字符串常量池有助于为Java运行时节省大量空间，虽然创建字符串时需要更多的时间。</li>
<li>当我们使用<strong>双引号创建一个字符串时</strong>，首<strong>先在字符串常量池中查找是否有相同值的字符串，如果发现则返回其引用，否则它会在池中创建一个新的字符串，然后返回新字符串的引用。</strong></li>
<li>如果使用<strong>new运算符创建字符串，则会强制String类在堆空间中创建一个新的String对象。</strong>我们可以使用intern()方法将其放入字符串常量池或从字符串常量池中查找具有相同的值字符串对象并返回其引用</li>
</ul>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">StringPool</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * Java String Pool example</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@param</span> args</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>&#123;</span><br><span class="line">        String s1 = <span class="string">"Cat"</span>;</span><br><span class="line">        String s2 = <span class="string">"Cat"</span>;</span><br><span class="line">        String s3 = <span class="keyword">new</span> String(<span class="string">"Cat"</span>);</span><br><span class="line"></span><br><span class="line">        System.out.println(<span class="string">"s1 == s2 :"</span>+(s1==s2));</span><br><span class="line">        System.out.println(<span class="string">"s1 == s3 :"</span>+(s1==s3));</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>



<h3 id="4-1-面试题"><a href="#4-1-面试题" class="headerlink" title="4.1 面试题"></a>4.1 面试题</h3><ul>
<li>下面语句创建了几个对象</li>
</ul>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">String str = <span class="keyword">new</span> String(<span class="string">"Cat"</span>);</span><br></pre></td></tr></table></figure>

<ul>
<li>在上面的语句中，可能创建1或2个字符串对象。如果池中已经有一个字符串<em>“Cat”</em>，那么堆中只会创建一个字符串<em>“str”</em>。</li>
<li>如果池中没有字符串字面量<em>“Cat”</em>，那么它将首先在池中创建，然后在堆空间中创建，因此将创建总共2个字符串对象。</li>
</ul>
<h3 id="4-2-作用"><a href="#4-2-作用" class="headerlink" title="4.2 作用"></a>4.2 作用</h3><ul>
<li>字符串的分配，和其他的对象分配一样，耗费高昂的时间与空间代价。JVM为了提高性能和减少内存开销，在实例化字符串常量的时候进行了一些优化。</li>
<li><strong>为 了减少在JVM中创建的字符串的数量，字符串类维护了一个字符串池，</strong>每当代码创建字符串常量时，JVM会首先检查字符串常量池。如果字符串已经存在池中， 就返回池中的实例引用。</li>
<li>如果字符串不在池中，就会实例化一个字符串并放到池中。Java能够进行这样的优化是因为字符串是不可变的，可以不用担心数据冲突 进行共享。</li>
</ul>
<p><strong>参考博客：</strong></p>
<p><a href="https://blog.csdn.net/itchuxuezhe_yang/article/details/89966303" target="_blank" rel="noopener">https://blog.csdn.net/itchuxuezhe_yang/article/details/89966303</a></p>

      
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